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Andy Thomas, managing director of England and Wales at Network Rail, said: "On very sunny days, rails in direct sunshine can be as much as 20C above air temperature, causing the steel to expand markedly, and could - if not carefully monitored and action taken - buckle, causing travel disruption.

"Our engineers and specialist extreme weather teams are monitoring track-side temperatures at vulnerable locations and will, if necessary, introduce temporary speed restrictions during the hottest part of the day to keep trains running, albeit more slowly than normal."